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My 1970 house was wired with aluminum branch circuits which I pig-tailed 35 years ago. I want to test the circuits by pulling 13 A from each receptacle and observing the voltage drop. There are six 15-A circuits 12 AWG AL and four 20-A 10 AWG AL. I have added two more 20-A circuits 12 AWG CU.

How should I proceed? Should I first take the cover off the panel and test the voltage drop on the incoming legs or should I proceed to test the receptacles?

The electrical panel is a GE with four 2-pole 240 V breakers in the top (30 A, 40 A, 50 A, and 60 A, for clothes dryer, a/c, kitchen range, and branch feeder) and a lower section with slots for two columns of eight 1/2-inch breakers fed by a 60-A 'main' breaker. The panel is in the attached garage.

The panel is now full because I have two GFCI breakers (1 inch each) and I have one 2-pole 50 A breaker for the whole house surge protector (Eaton). The house has no AFCI breakers. If I want this, I would have to use AFCI receptacles.

Preliminary results with a 13-A current draw (1600 W hair dryer):

Garage: one 15-A circuit goes from 124 V to 120 V, and one 20-A CU circuit also goes from 124 to 120. Another 15-A receptacle goes from 121 V to 113 V.

Farthest bedroom from garage: 15-A receptacle 122.7 V to 112.4 V.

  • Bad idea. Houses aren't designed for stress tests. Anyway, what do you plan to do with the test results if something "fails" ? – Carl Witthoft Nov 17 '16 at 14:43
  • I really would not call this a stress test. I am not drawing enough current to trip breakers. In the past we used a vacuum cleaner on these 15-A circuits wired with 12 AWG AL and I would bet it pulls about the same current as the hair dryer. In this house the two bathrooms are on the same 15-A circuit. When a hair dryer is used in one bathroom the lights would dim in the other, or at least did so with incandescent bulbs, not sure if it still does with LEDs. – Jim Stewart Nov 17 '16 at 18:51
  • Now I try to use the vacuum on one of the 20-A CU circuits which I added. I use an extension cord rather than unplug the vacuum and re-plug it in each room being cleaned. – Jim Stewart Nov 17 '16 at 18:59
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The distance and wire size would be needed to see if things are good. The formula is Voltage drop= 2K x L X I / Cm ... K for aluminum is 21.2 You will need to measure the lenth (L) and the current (I). You did not state the wire size so here are some values of circular mills (Cm). #14=4110, #12=6530, #10=10380, #8=16510, #6=26240, #4=41740, #3=5260. Doing the math after making the measurements you can see if the measured values are close to the calculated values. If your measured voltage drop exceeds the calculated values you may have some pigtails or connections that need to be cleaned and terminated with fresh anti oxide compound. remember their can be calibration issues with the amp reading and voltage depending on the conditions. If all your measurements are within a few percentage points of the calculated value every thing is probably fine, If you find 1 that has a higher voltage drop than the average calculation that may be a place to investigate.

  • Assuming the unit of L is in ft, the voltage drop in 50 ft of 12 AWG solid AL carrying 13 A would be 2 (21.2)(13 )(50)/6530 = 4 V. I see 10 V drop in a circuit which I could estimate as 50 ft. I guess I will check this circuit. – Jim Stewart Nov 17 '16 at 4:36
  • OK so the formula and constants in the answer above gives for the resistance per foot of aluminum house wiring is: 12 AWG 0.0065 ohm/ft and 10 AWG 0.0041 ohm/ft. – Jim Stewart Nov 17 '16 at 14:15
  • I agree w/ Ed that you're unlikely to achieve the measurement accuracy required. – Carl Witthoft Nov 17 '16 at 14:45
  • There is inaccuracy in determining the length of wire in the 15-A circuit which is showing the 10 V drop when carrying 13 A. There are 8 receptacles and one closet light (at the end) on that circuit in three different bedrooms! I might find that the length of wire is more like 100 ft. I could open up each one in turn moving from closest to farthest from the panel and check each one by stabbing through the insulation or just redo the splices with the AlumiConn connectors. – Jim Stewart Nov 17 '16 at 22:10
  • Wow 10v drop is huge in residential wiring over 2x the code requirement. I would leave the load in place at the furthest outlet and check each one to see if they are all dropping some voltage or if you find one that drops a few volts and tackle the worst ones first. Anything more than a few 1/10's of a volt should be cleaned and anti oxide compound applied prior to a new aluminum rated wire nut at the splice. – Ed Beal Nov 17 '16 at 23:36
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Following Ed Beal's suggestion I measured the voltage at each receptacle under no load and with a 13 A load at the farthest receptacle (no. 8). I also checked the drop at the breaker (screw not wire).

Receptacle Volts NL Volts 13 A load

Breaker 123.1 121.9

1 122.3 119.7

2 122.2 119.0

3 122.1 117.3

4 123.5 117.0

5 122.6 116.6

6 123.6 114.2

7 123.6 114.2

8 123.4 114.0

My proposed course is to redo splices at receptacles 3 and 6 and retest.

  • This is going to take more analysis than I can do right now. There is actually a 9th receptacle on the circuit and it gave about the same results as 8. As a trial I redid receptacle 6 with fresh Penetrox and new wire twist nuts, but it made no difference in the voltage drop. However, the box contained only one cable so it is a terminus and the boxes are not connected in the order I had assumed. Presumably box 6 is fed by box 5 and I will do that next with AlumiConn. Oh and the original work did not provide enough conductor--extends maybe two inches. – Jim Stewart Nov 18 '16 at 20:41
  • Redid pigtail on receptacle 5 with AlumiConn connectors. No change in voltage loss. I will take each receptacle in turn. There were two cables in this box. Presumably one of them feeds rec 6 across a wall. – Jim Stewart Nov 20 '16 at 19:04

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